Looking all the way back at the original post, it sure sounds like the wrong spark plug. As in too long. As in the piston is smacking the plug electrode.
Barring that, I'd check VERY carefully that the keyways on the top of the crankshaft and the flywheel align perfectly. There is a key in there to achieve that alignment, but only the proper torque on the flywheel nut will secure it sufficiently to the crankshaft. If the torque is too low, the flywheel will shift the very first time the spark plug fires, and that key will shear. Then your timing is gone, and that engine will never start.
Barring that, I'd check VERY carefully that the keyways on the top of the crankshaft and the flywheel align perfectly. There is a key in there to achieve that alignment, but only the proper torque on the flywheel nut will secure it sufficiently to the crankshaft. If the torque is too low, the flywheel will shift the very first time the spark plug fires, and that key will shear. Then your timing is gone, and that engine will never start.